1. Field of the Invention
The present device is directed generally to printed circuit boards. More specifically, the present invention is directed to interconnecting two or more signal layers of a multilayer printed circuit board while maintaining a well coupled signal return path to reduce signal noise.
2. Background of the Invention
Ground inductance is an industry-recognized source of ground bounce or noise. In the presence of parasitic capacitance, the ground circuit can even resonate causing extraordinary ground noise to be developed, with this effect being proportional to frequency.
Known methods are directed to minimizing ground inductance and limiting noise. Examples of various techniques may be found in U.S. Pat. No. 4,679,872, issued to Coe, and U.S. Pat. No. 5,808,529, issued to Hamre. U.S. Pat. No. 3,739,469, issued to Dougherty, Jr., the disclosure of which is hereby incorporated by reference, discloses the fabrication of a multilayered printed circuit board in which concentric through-holes (via holes) extend between layers to provide a higher density of via holes in the board. However, with increased frequencies, existing ground interconnect methods have proven inadequate and exhibit relatively high inductance. Known methods do not adequately address the need for coupling of the ground planes in a printed circuit board so as to minimize ground inductance, especially at high frequencies, such as at system clock frequencies on the order of 500 MHz or greater, or at direct frequencies on the order of 1 GHz or greater. Additionally, prior practices have become increasingly expensive and ineffective, mostly because of the incomplete coupling of the signal to the ground return path.
For example, consider a simple printed circuit board having a layer with a signal net that runs from one location on the board to another location on the board. On an adjacent layer to the signal net is a ground plane. As a signal current propagates down the signal net to its load on the board, the ground current returns to the driver on a ground plane. In DC, the ground current flows through the ground plane, spreading out over the board and taking the lowest ohmic path back to the driver. However, for an AC signal, under the transmission line principle, as the frequencies increase the ground current is channeled to flow in the shadow of the signal current. As a result, even though there is a choice of many different paths where the ground current may spread out, the AC return current is induced to concentrate in the shadow of the signal current.
Signal quality deteriorates whenever the ground return path is interrupted. In a coaxial cable, if a break in the shield existed, the break would radiate. In a printed circuit board, if there is a gap in a single ground plane (such as a crack or a via or a split of the plane into separate analog and digital planes), a high frequency signal crossing the gap would have its induced ground return current interrupted. The ground return current cannot flow in the shadow of the signal. The current would still return, but is forced to take an alternative and often circuitous route, forcing the ground return current to be separated from the signal current.
For example, a multilayered printed circuit board may have two or more ground planes and a signal current may have to be routed on more than one layer. When there are two or more ground planes on the board, it is advantageous to interconnect the ground planes so as not to interrupt the ability of the return current to remain in the shadow of the signal current. Conceptually, the ground plane is interconnected every time a through-hole technology part is installed on a board. For example, a DIP-type part can be inserted in a through-hole in the board and join together the ground planes at that location. Multiple parts placed into holes can result in a xe2x80x9cstitching togetherxe2x80x9d of the ground planes. However, this approach can result in the ground interconnections being separated from the signal vias.
The physical separation of the ground vias from the signal via can result in a radiation problem manifest in reduced AC signal quality. The transmission line properties can deteriorate such that cross-talking can occur and the ground can become noisy.
Exemplary embodiments of the present invention are directed to providing a signal net on a printed circuit board, such as boards having multiple ground plane, wherein a return current ground path remains coupled to the signal path.
In accordance with exemplary embodiments, a via on a printed circuit board having a circuit has a first interconnect and a second interconnect located about at least a portion of the first interconnect. The second interconnect connects to ground of the circuit and is coaxial and substantially concentric with the first interconnect and inductively coupled with the first interconnect.
A method of electrically interconnecting multiple layers on a printed circuit board to provide a continuous ground circuit for a signal is provided. The method connects a first layer and at least a second layer to a via disposed in a through-hole of a printed circuit board. The via has a first interconnect and a second interconnect located about at least a portion of the first interconnect, the second interconnect being coaxial and substantially concentric with the first interconnect and inductively coupled. The second interconnect is connected to ground of the circuit.
Exemplary embodiments connect the grounds from one printed circuit layer to another printed circuit layer, can reduce ground inductance in electrical circuits, and provide coupling that quiets the ground, even at UHF, while minimizing cost. The layers to be interconnected can be located in a single printed circuit board or in a monolithically integrated set of two or more printed circuit boards. Exemplary embodiments can attain a high speed interconnection by allowing the ground return path for a circuit carried on multiple layers of a multilayer printed circuit board to remain coupled to the signal, thereby lowering the ground inductance and maintaining the signal integrity.